By the material of the English language the article considers the question of drawing boundaries between linguistic units having a number of similar and opposite characteristics. As a result of the analysis of three controversial cases of defining boundaries in language material, on which there hasn’t been a consensus for more than half a century, the author concludes that it is necessary to take into account smooth transition from one phenomenon to another in the language system as a whole and justifies expediency to recognize the existence of intermediate transitional zones between linguistic phenomena.